5강. 오비탈 전자배치 규칙①⎥쌓음 원리⎥가림 효과⎥침투 효과✍🏻

Main ideas / concepts covered

• Orbital concept in the modern atomic model

  • Electrons occupy orbitals based on their energy levels (not just by the Bohr model’s shells).
  • Filling starts with the lowest-energy orbitals, using the typical energy order:
    • (1s \rightarrow 2s \rightarrow 2p \rightarrow 3s \rightarrow 3p \rightarrow 3d) (and so on)

• Example: Electron configuration of magnesium (Mg)

  • Magnesium has 12 electrons.
  • Filling by orbitals:
    • (1s): 2 e⁻
    • (2s): 2 e⁻
    • (2p): 6 e⁻ (three (2p) orbitals, each holds 2 e⁻)
    • (3s): 2 e⁻
  • Electron configuration:
    • (1s^2, 2s^2, 2p^6, 3s^2)
  • This arrangement is called electron configuration.

• Rule for filling orbitals: Aufbau principle

  • When filling orbitals, electrons occupy lower-energy orbitals first.
  • This is the Aufbau (battle) principle.

• Why orbital energies depend on quantum numbers

  • Principal quantum number (n)
    • Larger (n) → orbital is larger
    • Electron is, on average, farther from the nucleus
    • Farther electron experiences weaker attraction, so it has higher energy and is less stable
    • Therefore: higher (n) → higher energy
  • Azimuthal quantum number (l) (orbital shape)
    • If (n) is the same but orbital shapes differ, energy can differ due to:
      • Shielding
      • Penetration
    • This matters mainly in multi-electron atoms.

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Detailed methodology / instructions (how to determine filling order & compare energies)

1) Determine electron configuration (orbital filling order)

• Use the Aufbau principle:
  • Fill orbitals in increasing energy order.
• The lecture emphasizes a common high-school memory rule:
  • Remember that (4s) comes before (3d).
• Deeper exceptions beyond this level are considered less necessary for typical coursework.

2) Compare orbital energy when principal quantum number (n) changes

• Rule:
  • Smaller (n) → lower energy (more stable)
  • Larger (n) → higher energy (less stable)
• Reasoning:
  • Larger (n) increases orbital size and reduces electron–nucleus attraction.

3) Compare orbital energy when (n) is the same but (l) (shape) differs

• Hydrogen-like (one electron) case
  • Only (n) matters.
  • If (n) is the same (e.g., (2s) vs (2p)):
    • energies are equal
• Multi-electron atoms (2+ electrons)
  • Electron–electron interactions change the energies via:
    • Shielding effect
      • Inner electrons shield outer electrons from the nucleus’s full positive charge.
      • Example: electrons in (1s) influence how strongly (2s) and (2p) electrons feel attraction.
    • Penetration effect
      • Some orbitals allow electrons to be found closer to the nucleus more often.
      • The lecture contrast:
        • (2s): more probability closer to the nucleus → can be more stable (lower energy) than (2p)
        • (2p): electron region is, on average, farther → tends to be higher energy than (2s)
  • Net takeaway:
    • For the same (n), orbitals with smaller (l) tend to be lower in energy.

4) Determine orbital energy order in multi-electron atoms (practical comparison rule)

• Because tracking shielding/penetration for every case is complex, the lecture suggests a fast method:
• Compare using ((n + l)):
  • Larger ((n + l)) → higher energy
• Examples:
  • (4s): (n=4), (l=0 \Rightarrow n+l=4)
  • (3d): (n=3), (l=2 \Rightarrow n+l=5)
  • Since (5>4), (3d) has higher energy than (4s) → (4s) fills first
  • (3p): (n=3), (l=1 \Rightarrow n+l=4) (same as (4s))
    • Then, the lecture indicates the one with smaller (n) is lower energy (used to decide between (3p) and (4s)).

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Summary of the energy-ordering lesson

• Hydrogen (single electron)
  • Energy depends only on (n).
  • Same (n) → same energy regardless of (s/p/d) shape.
• Multi-electron atoms (2+ electrons)
  • Energy depends on both (n) and orbital shape ((l)), due to:
    • shielding
    • penetration
  • Common comparison method:
    • Use (n+l) to estimate which orbital is higher/lower energy.
  • High-school reminder:
    • (4s) comes before (3d).

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Speakers / sources

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